1. Field of the Invention
The present invention relates to a circuit for removing pulse noise components superposed on a video signal delivered from a solid-state image pickup device having pixel defects.
2. Description of the Prior Art
In image pickup devices such as video cameras, solid-state pickup devices are often used in the optoelectric conversion unit. When the output signal of the solid-state image pickup device is displayed by a monitor device and the video signal is observed, pixel defects known as white spots or black spots are sometimes observed in the picture. Such pixel defects are very obvious even if they are small in size, and pickup devices having pixel defects cannot be used as a commercial product, so that the production yield is lowered. Also, pixel defects can occur even after shipping of products. These problems are peculiar to the solid-state image pickup device, and present serious obstacles to their use.
Hitherto, as the means for solving the above problems, in a first example, as shown in U.S. Pat. No. 4,473,845, onset positions of pixel defects are stored in a PROM or the like, and the pixel defects are corrected by using the preceding and succeeding pixels on the scanning line at a timing when the output of the solid-state image pickup device corresponds to the stored position in the PROM.
In this first prior art example, the PROM is required as the means for storing the position of pixel defects. However, since the position of the pixel defects occurring in the solid-state pickup device varies from device to device, it is necessary to investigate the pixel defect position in each device and prepare a corresponding PROM, which requires too much labor. Further, this method is useless for pixel defects occurring after shipping of products.
As a second example, as disclosed in the Japanese Laid-open Patent Application Sho. 61-261974, pixel defects are detected by comparison of a pixel of notice with adjacent pixels positioned in the scanning line direction or in the direction vertical to the scanning line direction with respect to the pixel of notice, and the signal value of the pixel of notice is replaced by the value of one preceding pixel or the mean value of the preceding and succeeding pixels, so that the detected pixel defect is corrected.
In the second prior art example, when detecting pixel defects, attention is paid only to the scanning line direction or the direction vertical to the scanning line direction. However, since no comparison is made with respect to pixels in oblique directions, fine line segments in oblique directions to the scanning line are detected as pixel defects, and all line segments in oblique directions may be removed. Also, when correcting pixel defects, the pixel value is replaced by one preceding pixel or the mean value of preceding and succeeding pixels, but if there is a pixel defect on a fine line segment extending in the direction vertical to the scanning line direction, the line segment may be cut off as a result of correction.